It is desirable, if not mandatory, to provide submersible electric motors, such as used to provide the power for a submersible pump in the environment of a well, with protection for sudden high voltage surges. Without such protection, if lightning, for example, were to induce a voltage surge into the electrical wires leading down to the pump, the high voltage created thereby would be carried down to the motor and potentially burn up the windings thereof.
One of the easier known, but less effective, methods of protecting such motors from electrical surges is to provide a conventional surge protection or lightning arresting device externally of the motor. Most conveniently, these are provided above ground and tied, for example, to the incoming line of the control box for the system. However, being remote from the motor which could, for example, be several hundred feet away at the bottom of a well, protection against surges induced between the control box and the motor is not afforded.
As an alternative, surge suppressors have been provided along the power line in the well near the motor. While being closer to the motor and therefore affording better protection than similar devices located at the control box, such a system is not without problems of its own. In particular, such items are rather costly to manufacture and install. First, the surge suppressor must be totally encapsulated and sealed from the liquid environment of the well. In addition, the encapsulated suppressor must be sufficiently small to be received in the cramped confines of the well environment where there is often barely space for the power wires let alone the surge suppression system. Moreover, proper installation of such a device is tedious and without careful attention, the suppression device could easily be damaged thereby leaving the motor unprotected.
Another approach is to attempt to position a surge suppressor directly within the motor housing at exemplified by U.S. Pat. No. 3,849,704. In the device shown in that patent, a bore must be drilled in the motor housing and then a lightning arrestor, of an arc-extinguishing, gas expulsion type, is press fit into the bore. The outer surface of the lightning arrestor acts as one electrode and in the event of a voltage surge, spark gaps in the arrestor break down shorting the voltage surge through the motor housing. The heat from the arcing causes a disc which is adjacent the gap to generate a gas which extinguishes the arc sb that normal motor operation can continue.
There are many problems inherent with such a system. For example, the device is costly and tedious to manufacture, requiring several operational steps. First, hole must be precisely bored in the housing to accept the press fit of the lightning arrestor to assure the electrical connection. Then an elaborate seal is necessary to assure that potting material used to hold the motor windings, terminals and the like in place is not allowed to seep into the chamber of the lightning arrestor.
From an operational standpoint, the gas expulsion type of lightning arrestor is undesirable in that upon each voltage surge, gas is emitted into the arrestor chamber which will eventually build up enough pressure to cause the housing to fracture and the arrestor to fail. Such failure cannot be detected by the user and therefore the next voltage surge can damage the motor. Moreover, as the gas emitting disc ages through use and its strength diminishes, it will not react as quickly causing inefficient and detrimental operation. Finally, when the disc is no longer able to emit gas, if the housing has not already fractured by that time because of the gas build up, the next voltage surge can permanently short out the motor. It is highly likely at that time that the whole motor, rather than merely the worn out arrestor, would have to be replaced.